Onium hydroxides, such as quaternary ammonium hydroxides including tetramethylammonium hydroxide (TMAH) and tetraethylammonium hydroxide (TEAH), are strong organic bases that have been known for many years. Quaternary ammonium hydroxides have found a variety of uses including use in zeolite manufacture and polymer manufacture. Aqueous solutions of quaternary ammonium hydroxides, particularly TMAH solutions, have also been used extensively as a developer for photoresists in printed circuit board and microelectronic chip fabrication. For a variety of reasons, it is desirable to minimize the overall amount of developer used in printed circuit board and microelectronic chip fabrication. One way to minimize the overall amount of hydroxide developer is to reuse the waste developer. Reusing developer reduces the amount lost and decreases disposal problems.
Waste developer contains impurities including ionic impurities and nonionic impurities. Ionic impurities include various metal cations such as sodium, potassium, zinc, nickel, aluminum, copper and calcium; and anions such as halides, nitrates, nitrites, carbonates, carboxylates, sulfates. Nonionic impurities include photoresists, surfactants, amines and numerous other organic molecules. Waste developer also contains relatively low concentrations of the hydroxide developer. Accordingly, there remains a continuing need to effectively recover hydroxide developer in a useable form so that it may be reused thereby minimizing the overall amount of developer used in printed circuit board and microelectronic chip fabrication.
U.S. Pat. No. 4,714,530 (Hale et al) describes an electrolytic process for preparing high purity quaternary ammonium hydroxides which utilizes a cell containing a catholyte compartment and an anolyte compartment separated by a cation-exchange membrane. The process comprises charging an aqueous solution of a quaternary ammonium hydroxide to the anolyte compartment, adding water to the catholyte compartment, and passing a direct current through the electrolysis cell to produce a higher purity quaternary ammonium hydroxide in the catholyte compartment which is subsequently recovered. The '530 patent also describes an improvement which comprises heating the quaternary ammonium hydroxide at an elevated temperature prior to charging the hydroxide to the anolyte compartment of the electrolytic cell.
U.S. Pat. No. 4,938,854 (Sharifian et al) also describes an electrolytic process for purifying quaternary ammonium hydroxides by lowering the latent halide content. The electrolytic cell may be divided into an anolyte compartment and a catholyte compartment by a divider which may be an anion or cation selective membrane. The cathode in the catholyte compartment comprises zinc, cadmium, tin, lead, copper or titanium, or alloys thereof, mercury or mercury amalgam. Japanese Kokai Patent No. 60-131985 (1985) (Takahashi et al) describes a method of manufacturing a high purity quaternary ammonium hydroxide in an electrolysis cell which is divided into an anode chamber and a cathode chamber by a cation exchange membrane. A quaternary ammonium hydroxide solution containing impurities is charged to the anode chamber and a direct current is supplied between two electrodes after water has been charged to the cathode chamber. Purified quaternary ammonium hydroxide is obtained from the cathode chamber. The purified quaternary ammonium hydroxide contains reduced amounts of alkali metals, alkaline earth metals, anions, etc.
U.S. Pat. Nos. 5,439,564 and 5,545,309 (Shimizu et al) relate to methods of processing waste liquid containing an organic quaternary ammonium hydroxide by contacting the waste liquid with a cation-exchanging material, eluting organic quaternary ammonium cations from the cation-exchanging material, and electrolyzing the eluate in a two chamber electrolytic cell equipped with an anode, cathode and cation-exchanging membrane. Organic quaternary ammonium hydroxide is obtained from the cathode chamber of the electrolytic cell.